This application claims the priority of Japanese application No. 9-042009, filed Feb. 26, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an axial piston displacement type machine having pistons arranged so as to reciprocate in a drive shaft direction and, more particularly, to an axial piston type liquid pump suitable for pressurizing and transporting liquid and an axial piston type liquid motor for driving an output shaft by a pressurized liquid.
An example of conventional axial piston displacement type machines is disclosed in MECHANICAL ENGINEER'S HANDBOOK edited by Japan Association of Mechanical Engineers (1991), B 5, FLUID MACHINE, Page 188 FIG. 420(c), which is a swash plate type liquid pump. The swash plate type liquid pump comprises a fixed cylinder block having a plurality of pistons inserted in cylinders formed therein, a swinging plate linked with the pistons and prevented from rotating by a rotation preventing mechanism (not shown) and a swash plate which is arranged between the swinging plate and a fixed frame (not shown) and in contact with the swinging plate through bearings therebetween. Rotation of the swash plate swings the swinging plate, and the swinging motion of the swinging plate reciprocates each of the pistons in the cylinders.
Other examples of the conventional axial piston displacement type machines are disclosed in MECHANICAL ENGINEERING HANDBOOK B 5, FLUID MACHINE, Page 188 FIG. 420(a) and Page 191 FIG. 441, one of which is a bent axis type axial piston liquid pump and the other is a bent axis type axial piston liquid motor. The pump or motor comprises a cylinder block having a plurality of pistons inserted in cylinders formed therein, an input or output shaft the axis of which is inclined against the axis of the cylinder block and which is linked to the pistons, a rotation synchronizing mechanism connecting the shaft and the cylinder block to allow them to rotate in synchronism with each other and a frame (not shown) rotatably supporting the shaft and the cylinder block so that they are able to rotate about their axes, respectively. In the pump as shown in FIG. 430(a), rotation of the input shaft rotates the cylinder block and reciprocates the pistons in the cylinders and in the fluid motor as shown in FIG. 441, fluid supplied into the cylinders by operation of valves reciprocates the pistons and the reciprocation of the pistons rotates the output shaft.
In the above-mentioned conventional swash plate type liquid pump, the swinging plate and the frame do not rotate, however, the swash plate incorporated between the swinging plate and the frame rotates, so that two rotation-sliding portions exist at which the swash plate slides at a relatively large sliding speed under a relatively large load due to hydraulic pressure applied by pumping.
Further, in the above-mentioned conventional bent axis type axial piston liquid pump or motor, the frame which does not rotate and an flange portion of the input or output shaft which rotates slide at a relatively large sliding speed under a relatively large thrust load due to liquid pressure applied on the piston head.
Further, in a case where an opening area of each cylinder at a side of the valve plate is smaller than the cross-sectional area of each piston, a thrust load occurring in the cylinder block according to the difference in liquid pressure receiving area acts on the valve plate. The thrust load is relatively large and the cylinder block slides on the valve plate at a relative large sliding speed with such a relatively large thrust load.
The above-mentioned conventional swash plate type liquid pump, the bent axis type liquid pump and bent axis type liquid motor each have a common structure in which first and second members are arranged which do not effect relative rotating motion and effect only relative swinging motion according to rotation of the input or output shaft, the first member is engaged with a plurality of pistons at positions around an axis thereof, the second member has a plurality of cylinders formed therein nearly in parallel to and around an axis thereof and the pistons are slidably inserted in the cylinders to form a plurality of working chambers, respectively. In this construction, the rotation of the input or output shaft, the relative swinging motion of the first and second members and the reciprocation of the pistons are linked, whereby the fluid is pressurized or transported by driving the output shaft to rotate, or, on the contrary, the output shaft is driven by supplying a controlled pressurized fluid into the working chambers.
For example, in the conventional swash plate type liquid pump, the swinging plate is the first member, and the second member is a fixed member such as the cylinder block to which the frame and a cylinder head are fixed.
Both of the swinging plate and the cylinder block do not effect a relative rotation because of the rotation preventing mechanism provided for the swinging plate, but effect the relative swinging motion which is imparted to the swinging plate by the rotation of the swash plate caused by rotation of the input shaft integrated with the swash plate.
The rotation of the swash plate, caused by rotation of the input shaft swings the swinging plate relative to the cylinder block, and the swinging motion reciprocates the pistons, whereby the volume of each working chamber is changed to pressurize and transport the fluid.
On the other hand, in the other conventional bent axis type fluid pump or fluid motor, the input or output shaft having a flange is the above-mentioned first member, and the cylinder block is the second member. Since both of them rotate together by the rotation synchronizing mechanism, they do not effect relative rotating motion, but effect relative swinging motion because the first and second members have the axes inclined to each other and rotate about the axes in synchronism with each other, respectively.
The plurality of pistons linked to a flange portion of the input or output shaft which is the first member through rods are slidably inserted in the cylinders formed in the cylinder block which is the second member around the axis of the cylinder block nearly in parallel to the axis to form a plurality of working chambers, respectively.
The rotation of the input or output shaft, the relative swinging motion between the flange of the shaft and the cylinder block and the reciprocating motion to increase or decrease the volume of each working chamber are linked, so that the bent axis type axial piston fluid pump pressurizes and transports the fluid by driving the input shaft to rotate and the bent axis type axial piston fluid motor supplys a controlled pressurized fluid into the working chambers to drive the output shaft to rotate.
As mentioned above, in the conventional axial piston displacement type machines, a rotation-sliding portion at which a sliding load and a sliding speed each are large exists at a portion that bears the thrust force. In a case where a slide bearing is provided for the rotation-sliding portion, there has been such a technical problem that the efficiency of the machine decreases due to a mechanical friction loss and the reliability also decreases because of occurrence of seizure.
On the other hand, in a case where a thrust roll bearing which has a relatively small frictional resistance is incorporated for the rotation-sliding portion, the above-mentioned decrease in efficiency and reliability can be improved to some extent, however, there has been still left such a problem to be improved that the roll bearing is restricted in making the life long because metal fatigue progresses and it also raises the cost because the number of parts increases.